This invention related to U.V.-absorbing agents suitable for cosmetic purposes.
U.V.-rays from sunlight have long-lasting action on the human skin. Rays of the 280 to 230 nm wavelength range cause sunburn or erythema, especially in fair-skinned persons. Even "browning" rays of 320 to 400 nm (U.V.-A), wavelength range, often regarded as desirable, can have permanently undesirable effects on the condition and appearance of the skin. These include pathological changes in the skin, e.g., light dermatoses. Especially in the case of long-term action, U.V.-A rays also cause more rapid aging of the skin exposed to light, especially facial skin, than of skin protected by clothing. Effective agents for protection against rays in the 280 to 320 nm (U.V.-B) range are known. See German Patent Specification No. 2,051,824, now U.S. Pat. No. 3,781,417.
Dianisoylmethane, which has been used as U.V. filter, absorbs U.V. rays up to about 380 nm. However, in the region below 330 nm, dianisoylmethane has limited absorption and, furthermore, is only sparingly soluble in conventional cosmetic oils and fats. Methoxy- and hydroxy-substituted benzophenones, also used as "broad band filters", have a very low fat solubility and exhibit satisfactory absorption behavior only up to about 340 nm.
The hydroxy-ketones of U.S. Pat. No. 3,123,647 also show a very low fat and oil solubility and do therefore not fulfill the requirements for a sun-protection agent for cosmetics. It has been found out that the U.V. absorbance of a certain U.V. absorbing compound physically measured in the usual way using a very low concentrated solution of the compound in a suitable solvent does not in all cases correlate with the sun-protection factor of a cosmetic composition containing this compound. Surprisingly it has been found that U.V. absorbing compounds which are not fully soluble in cosmetic compositions show less satisfactory sun-protection factors than is to be expected from the physically measured absorbance in solution.
With other words, the physically measured absorbance of a certain compound does not give any hint whether this compound will be an effective U.V. absorbing agent when it is incorporated in a cosmetic formulation. The completely different behavior of an absorbing compound in a cosmetic carrier compared with the physically measured absorption in low concentration in a solvent not suitable for cosmetics can be shown by the following experiment:
Dianisoylmethane, a structurally close sun-protection agent used in compositions of the art, shows a comparable good U.V. absorption like the compounds of this invention when measured in the normal way using a concentration of some mg/100 ml and a path length of 10 mm. However, using a path length of only about 20 .mu.m, which is realistic for the application on human skin, and a concentration of some g/100 ml in a typical cosmetic carrier the situation is completely different.
In FIG. 1 the extinction coefficients of dianisoylmethane and 4-isopropyldibenzoylmethane are plotted against the concentration in liquid paraffine. It is clearly shown that in the case of 4-isopropyldibenzoymethane there is a linear dependency of the extinction coefficient from the concentration. That means, that with rising concentrations very high sun-protection factors can be reached.
In the case of dianisoylmethane from the physically measured absorbance in low concentration about the same extinction as for 4-isopropyldibenzoylmethane should be expected. However, as FIG. 1 shows, on the one hand the extinction coefficients are much lower than expected and on the other hand there is no linear dependency from the concentration but already at low concentrations a plateau is reached. This means, that in the case of dianisoylmethane it is not possible to reach higher sun-protection factors by raising the concentration of the active compound in the formulation. This completely unsatisfactory result was reached although the compound was micronized and homogeneously distributed by ultrasonic treatment.
With only partly soluble U.V. absorbing compounds still other serious problems arise, for instance to achieve a homogeneous distribution of the compound in the composition and on the human skin. "Broad band filters" with good solubility and additionally good U.V. absorption which are suitable for the use in cosmetics have hitherto not been known.
Numerous U.V. stabilizers for polymers are known. However, these U.V. stabilizers as a rule are not suitable for cosmetic purposes since the requirements for a U.V. stabilizer in polymers are completely different from those of a U.V absorbing agent in cosmetics. To be used in cosmetics a compound must at least fulfill the following requirements: good U.V. absorption, good chemical stability, photostability, as little odor as possible, non-staining, tasteless, toxicologically unobjectionable, good toleration by skin and mucous membranes, i.e. non-irritant, non-sensitizing, non-allergenic, easily to be processed, good solubility in the solvents or bases usual for cosmetics, good compatibility with the usual bases, good stability of the finished product.
For a compound to be used in polymers most of these requirements are negligable, other requirements which do not play any role in cosmetics becoming more important, like for instance the ability to stop chain reactions in the polymer.
In practice this means that there are completely divided fields of the art; on the one hand the polymer specialist knowing all about the physical and chemical behavior of polymers; and on the other hand the specialist dealing with the complex and still widely unknown physiological and biochemical behavior and reactions of living tissue, the human skin.
Thus, although there are numerous U.V. absorbing agents already known, there is a continuing need for agents to protect human skin against the effects of harmful U.V. rays. Such agents should, in a suitable medium, contain an active material which absorbs the rays of 285 to 380 nm as completely as possible, is non-toxic and compatible with the skin, has good solubility in cosmetic oils and, especially, has high light stability.